A fiber-reinforced composite material in which carbon fibers, aramid fibers, glass fibers or the like are used as reinforcing fibers has been widely utilized for structural materials of aircrafts, vehicles or the like, or in general industries and sports such as a tennis racket, a golf club shaft and a fishing rod by utilizing high specific strength and high specific elasticity thereof. The forms of reinforcing fibers used therein may include a woven fabric made by using continuous fibers, a UD sheet in which fibers are pulled and aligned unidirectionally, a random sheet made by using cut fibers, a non-woven fabric and the like.
Generally, in case of the fabric made from continuous fibers or the UD sheet and the like, complicated layering steps such as layering at various angles of, for example, 0/+45/−45/90, because of anisotropy of the fibers, or and further plane-symmetrical layering for preventing warpage of a shaped product. This is one of factors that increase the cost for the fiber-reinforced composite material.
Thus, by using an isotropic random mat in advance, a relatively inexpensive fiber-reinforced composite material may be obtained. The random mat may be obtained by a spray-up method (dry production method) wherein spraying cut reinforcing fibers alone or spraying the cut fibers together with a thermosetting resin are performed at the same time into a mold, or a paper-manufacturing method (wet method) of adding previously cut reinforcing fibers into an aqueous slurry containing a binder, followed by paper-making. The dry manufacturing method requires a relatively small device and thus allows the random mat to be obtained at a lower cost.
In the dry production method, a method of cutting continuous fibers and simultaneously spraying the cut fibers is frequently used, and a rotary cutter is used mostly. However, when an interval between blades of the cutter is widened in order to increase a fiber length, the cut frequency is decreased and thus results in discontinuous discharge of the fibers from the cutter. For this reason, unevenness in fiber areal weight of the fibers in the mat locally occurs. Especially, when a mat with a low fiber areal weight of fibers is made, there is a problem in that unevenness in thickness becomes significant and thus surface appearance is deteriorated.
Meanwhile, another factor that increases the cost for the fiber-reinforced composite material is that a long time is required for molding. In general, the fiber-reinforced composite material is obtained by heating and pressurizing a material called a prepreg by an autoclave for 2 hours or more, in which the prepreg is obtained by impregnating a reinforcing fiber base material with a thermosetting resin in advance. There has recently been suggested an RTM molding method in which a reinforcing fiber base material not impregnated with a resin is set in a mold, and a thermosetting resin is poured thereto. This significantly shortens a time for molding. However, even though the RTM molding method is adopted, a time required for molding one part is 10 minutes or more.
Therefore, a composite material employing a thermoplastic resin as a matrix, in place of conventional thermosetting resin, has been spotlighted. However, the thermoplastic resin generally has a higher viscosity than the thermosetting resin, and thus has a problem in that a time for impregnating a fiber base material with the molten resin is long, and as a result, a tact time until molding is prolonged.
As a method for solving the foregoing problems, there is suggested a method called thermoplastic stamping molding (TP-SMC). This is a molding method in which chopped fibers impregnated with a thermoplastic resin in advance are heated up to a melting point or more or a flowable temperature or more of the resin, and are put into a part of a mold, and immediately, the mold is closed. In the method, the fibers and the resin are allowed to flow in the mold so as to form a product shape, followed by cooling to form a shaped product. In the method, since the fibers impregnated with the resin in advance are used, it is possible to mold in a short time of about 1 minute. A method of manufacturing a chopped fiber bundle and a molding material is disclosed in Patent Documents 1 and 2. However, the disclosed method employs a molding material called an SMC or a stampable sheet. In such thermoplastic stamping molding, fibers and a resin are largely flowed within a mold, and thus fiber orientation is disturbed. In a case of a random mat employing cut fibers, lack of isotropy is caused due to unidirectional fiber orientation. As a result, a development rate of physical property of an isotropic composite material is lowered by unidirectional orientation caused by the flow of the resin and the reinforcing fibers. Also, in the molding within the mold accompanying the flow of the reinforcing fibers and the matrix resin, especially, a mold temperature or a mold structure has to be investigated in order to secure stability in a thickness direction and a planar dimension of a shaped product. Thus, there is a problem in that it is difficult to adjust manufacturing conditions in mass production, and also it is difficult to make a thin-walled product. Meanwhile, for the composite material employing the thermoplastic resin as a matrix, there is suggested a technique in which a long-fiber pellet that contains reinforcing fibers is injection-molded. However, the long-fiber pellet also has a limitation on the length of the pellet. Further, there is a problem in that the reinforcing fibers in the thermoplastic resin are cut by kneading and thus the length of the reinforcing fibers may not be kept. Also, the molding method through such injection molding has a problem in that the reinforcing fibers are oriented, and thus isotropy may not be achieved.
Also, as a means for a thin-walled product without flow of fibers, there is suggested a method of manufacturing a prepreg in which a thin sheet is manufactured from reinforcing fibers through a paper-making method, and then the thin sheet is impregnated with a resin (Patent Document 3). In the paper-making method, since reinforcing fibers are uniformly dispersed in a dispersion liquid, the reinforcing fibers are in single fiber form.
(Patent Document 1) Japanese Patent Laid-Open Publication No. 2009-114611
(Patent Document 2) Japanese Patent Laid-Open Publication No. 2009-114612
(Patent Document 3) Japanese Patent Laid-Open Publication No. 2010-235779